HOW RELIABLE ARE CORONAL HOLE TEMPERATURES DEDUCED FROM OBSERVATIONS

Given the importance of the temperature at the base of the corona for the modeling of the solar wind, we investigate the range of temperatures which have been deduced from remote measurements in coronal holes, within a heliocentric distance of 1.6 R(s), and the accuracy to which these temperatures have been inferred. We also present results of an analysis of extreme ultraviolet (EUV) observations made simultaneously in three spectral lines at the limb in a polar coronal hole, with very little contamination from quiet region emission along the line of sight. The values most commonly quoted for temperatures below 1.6 R(s) extend from 7 x 10(5) K to 1.6 x 10(6) K. However, by carefully reviewing the assumptions that enter into these inferences, we find that, within 1.6 R(s), the temperature in a coronal hole is at or below 1.3 x 10(6) K. Our own analysis yields a temperature range of 7.8-9.3 x 10(5) K, between 1.02 to 1.07 R(s) for the coronal hole, and a distinctly different temperature range of 9.4 x 10(5)-1.2 x 10(6) K for the quiet regions bordering it. Inhomogeneities within the coronal hole itself contribute to a 14% variation in its inferred temperature. Our results show that, from data currently available, temperatures in the inner corona cannot be derived with an accuracy better than 20%, even when neglecting many sources of uncertainty such as instrument calibration, line of sight effects, departure from ionization balance, and inaccuracies of the atomic data. We also show in this study that the elemental abundance, which is one of the parameters that influence the temperature inference, can in turn be significantly constrained when intensity ratios from three spectral lines are used. We find that, within the wide range of values quoted so far in the literature, the larger values of the abundance of O[log (A(O)) = 8.96], Ne[log (A(Ne)) = 8.2], and Mg[log (A(Mg)) = 7.65] [relative to hydrogen when log (A(H)) = 12], yield a self-consistent temperature.